The cell cycle comprises S phase (DNA replication), M phase (mitosis), and two gap phases (G1 and G2 phases) between S and M phases. Checkpoints in the cell cycle ensure accurate progression through cell cycle stages, and include monitoring the state of DNA integrity, DNA replication, cell size, and the surrounding environment (Maller (1991) Curr. Opin. Cell Biol., 3:26). It is especially important for multi-cellular organisms to maintain integrity of genome, and there are multiple checkpoints that monitor the state of genome. Among them are G1 and G2 checkpoints prior to DNA replication and mitosis, respectively. It is crucial to repair or correct DNA damage before entering S phase, because once damaged DNA is replicated it often gives rise to mutations (Hartwell (1992) Cell, 71: 543). Progression through G1 and G2 checkpoints without repairing extensive DNA damage induces mitotic catastrophe and/or apoptosis.
Most cancer cells carry abnormalities in G1 checkpoint-related proteins such as p53, Rb, MDM-2, p16INK4 and p19ARF (Levine (1997) Cell, 88:323). Alternatively, mutations can cause overexpression and/or over-activation of oncogene products, e.g., Ras, MDM-2 and cyclin D, which reduce the stringency of G1 checkpoint. In addition to these mutations, excessive growth factor signaling can be caused by the overexpression of growth factors and can reduce the stringency of G1 checkpoint. Together with loss-of-function and gain-of-function mutations, continuous activation by growth factor receptors or downstream signal-transducing molecules can cause cell transformation by overriding the G1 checkpoint. A disrupted or abrogated G1 checkpoint contributes to higher mutation rates and the many mutations observed in cancer cells. As a result, most cancer cells depend on G2 checkpoint for survival against excessive DNA damage (O'Connor and Fan (1996) Prog. Cell Cycle Res., 2:165).
The G2 cell cycle checkpoint restricts the onset of mitosis until DNA replication and repair are complete. Malfunction of the G2 checkpoint would allow premature onset of mitosis prior to the completion of DNA replication and repair, producing daughter cells lacking a substantial portion of the genomic DNA or harboring mutations. Functions of the G2 checkpoint includes detecting DNA damage and generation of signal that can lead to cell cycle arrest when DNA damage is detected. The mechanism that promotes the cell cycle G2 arrest after DNA damage is believed to be conserved among species from yeast to human. In the presence of damaged DNA, Cdc2/Cyclin B kinase is kept inactive by phosphorylation of threonine-14 and tyrosine-15 residues on Cdc2 kinase; alternately, the level of Cyclin B protein may be reduced. At the onset of mitosis, Cdc25 phosphatase removes inhibitory phosphates from Cdc2/Cyclin B kinase, thereby activating Cdc2/Cyclin B kinase. The activation of Cdc2/Cyclin B kinase is equivalent to the onset of M phase.
In fission yeast, the protein kinase Chk1 is required for the cell cycle arrest in response to damaged DNA. Chk1 kinase acts downstream of several rad gene products and is modified by the phosphorylation upon DNA damage. The kinases Rad53 of budding yeast and Cds1 of fission yeast are known to conduct signals from unreplicated DNA. It appears that there is some redundancy between Chk1 and Cds1 because elimination of both Chk1 and Cds1 culminated in disruption of the G2 arrest induced by damaged DNA. Interestingly, both Chk1 and Cds1 phosphorylate Cdc25 and promote Rad24 binding to Cdc25, which sequesters Cdc25 to cytosol and prevents Cdc2/Cyclin B activation. Therefore Cdc25 appears to be a common target of these kinases implying that this molecule is an indispensable factor in the G2 checkpoint.
In humans, both hChk1, a human homologue of fission yeast Chk1, and Chk2/HuCds1, a human homologue of the budding yeast Rad53 and fission yeast Cds1, phosphorylate Cdc25C at serine-216, a critical regulatory site, in response to DNA damage. This phosphorylation creates a binding site for small acidic proteins 14-3-3s, human homologues of Rad24 and Rad25 of fission yeast. The regulatory role of this phosphorylation was clearly indicated by the fact that substitution of serine-216 to alanine on Cdc25C disrupted cell cycle G2 arrest in human cells. However, the mechanism of G2 checkpoint is not fully understood.